Maybe you can’t be bothered with those pesky calorie counts on fast-food menus, or perhaps that info isn’t posted at restaurants in your area, and instead think, “Hey, I’m sure I’m just eating [insert best estimate] calories!” You’re probably off. Way off, says a new study that looked at just how wrong diners are when it comes to estimating how many calories are in their fast-food meals.

Teens are the biggest underestimaters, says the study by the Harvard Medical School, published in BMJ, a journal of the British Medical Association. They underestimated calories by 34%, while parents who have kids in school were off by 23% and adults 20%, reports USA Today.

Which means, for example, if you’re guessing that a burger has 500 calories, it’ll have 600. And since I’m just making up that calorie count, heck, it probably has more.

The researchers surveyed 3,400 adults, teens and parents with kids who visited 89 fast-food joints, including McDonald’s, Burger King, KFC, Subway, Dunkin’ Donuts and Wendy’s. Participants were asked to estimate how many calories they were about to eat, and then researchers used their receipts to figure out the real number.

“These large underestimations show that diners don’t really know what they are eating in terms of calorie content, and they need this information to help guide their choices,” the study’s lead author says, underscoring the importance of knowing what you’re eating.

“They could get it from the company websites or in some other form in the restaurants, such as wall posters, napkins or cups, but soon they’ll be directly faced with it when they see it on the restaurant menu boards before they order their meal. Customers can already do this at McDonald’s — and in some cities,” he says.